Design and optimization of ultrasonic array transducers for medical imaging applications

Abstract

Currently array transducers are being studied with narrowband theory, however, it raises problems when the theory is used in medical imaging applications where broadband signals are used. This thesis investigates the properties of ultrasonic array transducers based on broadband analysis, in particular their radiation fields. And furthermore, the methods to design and optimize broadband sparse arrays are developed for both 1D and 2D arrays, which can be used to satisfy different application requirements. The radiation fields from the broadband arrays as well as the narrowband arrays are first analyzed and compared starting from the same mathematical model. Despite the simplification of the model, broadband fields show much more complicated behaviours than the narrowband ones, due to the coupling of spatial dimension and temporal dimension in broadband fields. This means that narrowband theory is not adequate to be used in broadband analysis. The efficient algorithms to calculate the broadband radiation field are then developed. Although the rigorous model for broadband transducers is available, it is not suitable for array calculation because of its low efficiency. The developed algorithms dramatically reduce the complexity of calculation procedure while the accuracy is still comparable with the rigorous model. Finite element method is used as alternative methodology to study arrays performance. The interaction between transducer and loading material, and cross coupling effect can be fully considered. The results show that when the cross coupling level is low, the radiation field matches well with the results from previously developed model. However, under high cross coupling level, the side lobe level dramatically increases, while the main lobe ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library iii Table of contents iv List of figures vi List of tables xii Nomenclatures xiii Chapter 1-v-5.4 Cross coupling in 1D array transducers 5.5 Simulation of array transducers radiation field 5.6 Conclusions Chapter 6 Sparse Array Transducers Design and Optimization 6.1 Introduction 6.2 Periodic and non-periodic sparse arrays 6.3 Introduction of Genetic Algorithms 6.4 GAs operators and parameters 6.5 Sparse array optimization 6.6 Conclusions Chapter 7 Experimental Validation 7.1 Synthetic method for array radiation field measurement 7.2 Experimental set up and procedure 7.3 Experimental results-1D arrays 7.4 Experimental results-2D arrays 7.5 Conclusions Chapter 8 Conclusions and Recommendations 8.1 Conclusions 8.